Aircraft air conditioning system and method of operating an aircraft air conditioning system

ABSTRACT

An aircraft air conditioning system including an ambient air supply line having a first end connected to an ambient air inlet and a second end connected to a mixer of the aircraft air conditioning system so as to supply ambient air to the mixer. A first compressor is arranged in the ambient air supply line and is configured to compress the ambient air flowing through the ambient air supply line. A bleed air supply line allows a flow of bleed air bled off from an engine or an auxiliary power unit therethrough. A bleed air turbine is driven by the bleed air flowing through the bleed air supply line and is coupled to the first compressor so as to drive the first compressor.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 14/859,610 filed Sep.21, 2015 and claims the benefit of the European patent application No.14 185 484.4 filed on Sep. 19, 2014 and of the European patentapplication No. 15 168 140.0 filed on May 19, 2015, the entiredisclosures of which are incorporated herein by way of reference.

BACKGROUND OF THE INVENTION

The present invention relates to an aircraft air conditioning system anda method of operating an aircraft air conditioning system.

So-called air-based air conditioning systems, as described for examplein DE 10 2008 053 320 A1 and US 2010/101251 A1 or DE 10 2010 054 448 A1and WO 2012/079756 A2, are usually used at present in commercialaircraft to air-condition the aircraft cabin. An aircraft airconditioning system serves to adequately cool or heat the aircraft cabinand to adequately supply fresh air into the aircraft cabin to ensurethat a prescribed minimum proportion of oxygen is present in theaircraft cabin. Further, the humidity within the aircraft cabin isadjusted by means of the aircraft air conditioning system. Moreover,during flight operation of the aircraft, the aircraft air conditioningsystem is used to pressurize the aircraft cabin so as to maintain thepressure within the aircraft cabin, compared to the ambient pressure atthe cruising altitude of the aircraft, at an elevated level.

Air-based air conditioning systems typically comprise an airconditioning unit, which is arranged, for example, in a wing root of theaircraft, and which is supplied with compressed process air that isgenerated by a compressor or bled off from an engine or an auxiliarypower unit (APU) of the aircraft. During flight operation of theaircraft, usually engine bleed air is used so as to supply the airconditioning unit of the aircraft air conditioning system withcompressed process air. During ground operation of the aircraft the airconditioning unit of the aircraft air conditioning system, however,typically is supplied with compressed process air from the auxiliarypower unit of the aircraft. In the air conditioning unit, the processair, upon flowing through at least one heat exchanger as well as throughvarious compression and expansion units, is cooled and expanded. Cooledprocess air exiting the air conditioning unit finally is supplied to amixing chamber where it is mixed with recirculation air recirculatedfrom an aircraft region to be air conditioned. The mixed air from themixing chamber, via respective mixed air lines, is supplied to theaircraft region to be air conditioned which may be divided into aplurality of air conditioning zones.

DE 10 2010 034 831 A1 and WO 2012/022758 A1 describe an aircraft airconditioning system, wherein compressed process air that is bled offfrom an engine of the aircraft is cooled by transferring heat to arefrigerant circulating through a refrigerant circuit. A turbine whichserves to expand the compressed process air prior to being directed intoan aircraft cabin is used to drive a compressor disposed in therefrigerant circuit.

SUMMARY OF THE INVENTION

The invention is directed at the object of specifying an aircraft airconditioning system which, at least during normal operation, allows acabin of an aircraft equipped with the aircraft air conditioning systemto be supplied with ambient air. Further, the invention is directed atthe object of providing a method of operating an aircraft airconditioning system of this kind.

An aircraft air conditioning system comprises an ambient air supply lineallowing a flow of ambient air therethrough. The ambient air supply linehas a first end connected to an ambient air inlet and a second endconnected to a mixer of the aircraft air conditioning system so as tosupply ambient air to the mixer. The first end of the ambient air supplyline may be directly connected to the ambient atmosphere, for example,via an opening provided in an aircraft outer skin via which the ambientair might be supplied to the ambient air supply line. The opening may,for example, be designed in the form of a scoop air inlet. It is,however, also conceivable that the ambient air supply line branches offfrom a further ambient air conducting line or a ram air channel.

The aircraft air conditioning system further comprises a firstcompressor arranged in the ambient air supply line and being adapted tocompress the ambient air flowing through the ambient air supply line. Inparticular, the first compressor is adapted to compress the ambient airflowing through the ambient air supply line to an elevated pressurewhich allows the ambient air, after being further processed, to besupplied to the mixer of the aircraft air conditioning system where itmay be mixed with recirculation air recirculated from an aircraft cabinbefore the mixed air generated in the mixer is supplied to the aircraftcabin so as to air-condition the aircraft cabin.

A bleed air supply line of the aircraft air conditioning system allows aflow of bleed air bled off from an engine or an auxiliary power unittherethrough. The bleed air supply line has a first end connected to anengine or an auxiliary power unit of the aircraft, wherein the supply ofbleed air into the bleed air supply line may be controlled by a suitablevalve. A second end of the bleed air supply line is connected to anaircraft environment, e.g., via a suitable bleed air discharge devicesuch as a ram air channel. Thus, the bleed air exiting the bleed airsupply line is discharged to the ambient atmosphere instead of beingsupplied to a mixer of the aircraft air conditioning system as inconventional aircraft air conditioning systems.

A bleed air turbine is driven by the bleed air flowing through the bleedair supply line. Thus, in the aircraft air conditioning system, thebleed air flowing through the bleed air supply line is supplied to thebleed air turbine, wherein the bleed air is expanded and thus cooled.Thereafter, the bleed air may be discharged to the aircraft environment,i.e., to the ambient atmosphere. The bleed air turbine may be designedas a two stage turbine or may have a variable nozzle design. Theoperation of the bleed air turbine then may be controlled in a flexiblemanner in dependence on the actual operating conditions of the aircraftair conditioning system.

The bleed air turbine is coupled to the first compressor so as to drivethe first compressor. For example, the bleed air turbine and the firstcompressor may be arranged on a common shaft. In the aircraft airconditioning system, the bleed air thus is used to drive the firstcompressor arranged in the ambient air supply line. The cabin of anaircraft equipped with the aircraft air conditioning system, however, atleast during normal operation of the aircraft air conditioning system,is supplied with ambient air which, via the first compressor, iscompressed to the required elevated pressure before being supplied tothe mixer of the aircraft air conditioning system.

In a preferred embodiment, the aircraft air conditioning system furthercomprises an ambient air branch line branching off from the ambient airsupply line upstream of the first compressor. The ambient air branchline may open into the ambient air supply line downstream of the firstcompressor and hence bypass the first compressor. The terms “upstream”and “downstream,” in this context, refer to the direction of flow ofambient air through the ambient air supply line. A second compressor maybe arranged in the ambient air branch line. The second compressor may beadapted to compress the ambient air flowing through the ambient airbranch line. Similar to the first compressor, the second compressor maybe adapted to compress the ambient air flowing through the ambient airbranch line to an elevated pressure which allows the ambient air, afterbeing further processed, to be supplied to the mixer of the aircraft airconditioning system. Basically, the second compressor, like the firstcompressor, may be driven by means of the bleed air turbine driving thefirst compressor or an additional bleed air turbine.

Preferably, however, the aircraft air conditioning system furthercomprises a cabin exhaust air line allowing a flow of cabin exhaust airtherethrough. A cabin exhaust air control valve may be provided in thecabin exhaust air line so as to control the flow of cabin exhaust airthrough the cabin exhaust air line. Further, the aircraft airconditioning system may comprise a cabin exhaust air turbine driven bythe cabin exhaust air flowing through the cabin exhaust air line. Hence,the cabin exhaust air flowing through the cabin exhaust air line may besupplied to the cabin exhaust air turbine, wherein the cabin exhaust airis expanded and thus cooled.

The cabin exhaust air turbine may be coupled to the second compressor soas to drive the second compressor. For example, the cabin exhaust airturbine and the second compressor may be arranged on a common shaft. Ina preferred embodiment of the aircraft air conditioning system, thecabin exhaust air discharged from the aircraft cabin thus is used todrive the second compressor arranged in the ambient air branch lineindependent of the first compressor arranged in the ambient air supplyline. As a result, a sufficient amount of compressed ambient air can besupplied to the mixer of the aircraft air conditioning system, evenduring flight operation of an aircraft equipped with the aircraft airconditioning system at high altitudes and thus at a low ambientpressure. Furthermore, by making use of the energy contained in thecabin exhaust air, the bleed air demand for driving the bleed airturbine and hence fuel burn penalty due to the bleed off of air from theaircraft engine can be reduced. The aircraft air conditioning systemthus can be operated with a particularly high energy efficiency.

The aircraft air conditioning system may further comprise a connectingline connecting the ambient air supply line to the ambient air branchline. In particular, the connecting line may connect the ambient airsupply line downstream of the first compressor to the ambient air branchline upstream of the second compressor. The terms “downstream” and“upstream,” in this context, refer to the direction of flow of theambient air through the ambient air supply line and the ambient airbranch line, respectively. Via the connecting line, the first compressorand the second compressor may be connected in series, i.e., ambient airat an elevated pressure which exits the first compressor may be suppliedto the second compressor so as to be further compressed.

An ambient air supply control valve may be provided which is adapted tocontrol the supply of ambient air to the second compressor. Inparticular, the ambient air supply control valve may be adapted tocontrol at least one of the flow of ambient air through the connectingline and the flow of ambient air through the ambient air branch line.For example, the ambient air supply control valve may be designed in theform of a three-way valve which is arranged in the region of an openingof the connecting line into the ambient air branch line. The ambient airsupply control valve then is adapted to connect the first compressor andthe second compressor either in series or in parallel, or to interruptthe supply of ambient air to the second compressor as desired.

The aircraft air conditioning system may further comprise an electroniccontrol unit which is adapted to control the operation of the cabinexhaust air control valve and thus the flow of cabin exhaust air throughthe cabin exhaust air line. Further, the electronic control unit may beadapted to control the operation of the ambient air supply control valveand hence the supply of ambient air to the second compressor.Preferably, the electronic control unit is adapted to control theoperation of the cabin exhaust air control valve and/or the operation ofthe ambient air supply control valve in dependence on an ambient airdemand of the aircraft air conditioning system. For example, theelectronic control unit may control the cabin exhaust air control valveand/or the ambient air supply control valve in such a manner that theamount of compressed ambient air exiting the second compressor isincreased in response to an increase in the ambient air demand of theaircraft air conditioning system. Similarly, the electronic control unitmay control the cabin exhaust air control valve and the ambient airsupply valve in such a manner that the amount of compressed ambient airexiting the second compressor is decreased or even reduced to zero inresponse to a decrease in the ambient air demand of the aircraft airconditioning system.

In a particularly preferred embodiment, the electronic control unit isadapted to control the operation of the cabin exhaust control valve andthe ambient air supply control valve in a synchronized manner independence on an ambient air control demand of the aircraft airconditioning system. This allows a particularly efficient operation ofthe second compressor, since it is ensured that the cabin exhaust airturbine is supplied with a sufficient amount of cabin exhaust air inorder to provide the required amount of driving energy for driving thesecond compressor so as to compress the ambient air supplied to thesecond compressor in a controlled manner to the desired elevatedpressure.

A compressed air recirculation line may connect a portion of the ambientair supply line which extends downstream of the first compressor with aportion of the ambient air supply line which extends upstream of thefirst compressor. The terms “upstream” and “downstream,” in thiscontext, refer to the direction of flow of the ambient air through theambient air supply line. Thus, compressed air exiting the firstcompressor, via the compressed air recirculation line, may berecirculated to the first compressor to be compressed again. This allowsan improvement of the heating performance of the aircraft airconditioning system in case the aircraft air conditioning system shouldbe used for heating purposes. A recirculation control valve may beprovided for controlling the flow of compressed air exiting the firstcompressor through the compressed air recirculation line back to thefirst compressor.

The aircraft air conditioning system may further comprise a reheaterwhich is arranged in the cabin exhaust air line. In particular, thereheater may be arranged in the cabin exhaust air line upstream of thecabin exhaust air turbine. The term “upstream,” in this context, refersto the direction of flow of the cabin exhaust air through the cabinexhaust air line. By heating the cabin exhaust air flowing through thecabin exhaust air line prior to being supplied to the cabin exhaust airturbine, the driving energy potential of the cabin exhaust air turbinecan be enhanced. Moreover, the risk that free water is present in thecabin exhaust air at an inlet and/or an outlet of the cabin exhaust airturbine can be reduced. Preferably, the reheater is thermally coupled tothe bleed air supply line. As a result, the cabin exhaust air flowingthrough the cabin exhaust air line, in the reheater, can be heated bythe transport of heat energy from the bleed air flowing through thebleed air supply line to the cabin exhaust air flowing through the cabinexhaust air line.

The aircraft air conditioning system may further comprise a precoolerwhich is arranged in the bleed air supply line and which serves to coolthe bleed air flowing through the bleed air supply line. Preferably, theprecooler is thermally coupled to a ram air channel. The bleed airflowing through the bleed air supply line, in the precooler, then can becooled by the transport of heat energy from the bleed air flowingthrough the bleed air supply line to ram air flowing through the ram airchannel.

The reheater for re-heating the cabin exhaust air flowing through thecabin exhaust air line may be thermally coupled to the bleed air supplyline at different positions. In a first embodiment of the aircraft airconditioning system, the reheater is thermally coupled to the bleed airsupply line upstream of the precooler arranged in the bleed air supplyline. In a second embodiment of the aircraft air conditioning system,the reheater is thermally coupled to the bleed air supply linedownstream of the precooler arranged in the bleed air supply line. Theterms “upstream” and “downstream,” in this context, refer to thedirection of flow of the bleed air through the bleed air supply line. Byarranging the reheater in thermal contact with the bleed air supply lineupstream of the precooler, the bleed air consumption of the overallsystem can be reduced. Arranging the thermal coupling between thereheater and the bleed air supply line downstream of the precoolerarranged in the bleed air supply line, however, allows a reduction ofthe ram air flow through the ram air channel. Hence, the architecture ofthe aircraft air conditioning system can be adapted in dependence ofwhether either a minimized bleed air consumption or a minimized ram airconsumption of the reheater is desired.

The aircraft air conditioning system may further comprise an ambient aircooler which is arranged in the ambient air supply line and which servesto cool the ambient air flowing through the ambient air supply lineprior to being supplied to the mixer of the aircraft air conditioningsystem. Preferably, the ambient air cooler is arranged in the ambientair supply line downstream of the first compressor, wherein the term“downstream,” in this context, refers to the direction of flow ofambient air through the ambient air supply line. The ambient air coolermay be thermally coupled to the ram air channel. The ambient air, in theambient air cooler, then may be cooled by the transport of heat energyfrom the ambient air flowing through the ambient air supply line to ramair flowing through the ram air channel.

In case the aircraft air conditioning system is provided with both, aprecooler arranged in the bleed air supply line and an ambient aircooler arranged in the ambient air supply line, the precooler and theambient air cooler may be arranged in the ram air channel either inparallel or in series. In case a parallel arrangement of the precoolerand the ambient air cooler in the ram air channel is desired, theprecooler and the ambient air cooler, for example, may be arranged intwo parallel ram air channel branches which each have ram air flowingtherethrough independent from each other. This configuration ensuresthat both, the precooler and the ambient air cooler, are supplied withram air having the same temperature and hence the same cooling capacity.However, in case a simple and hence lightweight design of the ram airchannel is desired, it is also conceivable to arrange the precooler andthe ambient air cooler in the ram air channel in series, i.e., the ramair may first be directed through the ambient air cooler and onlythereafter through the precooler or vice versa.

The aircraft air conditioning system may further be provided with acondenser which may be arranged in the ambient air supply line. Inparticular, the condenser may be arranged in the ambient air supply linedownstream of the ambient air cooler, wherein the term “downstream,” inthis context, refers to the direction of flow of ambient air through theambient air supply line. The condenser may be thermally coupled to thebleed air supply line. This allows the ambient air, in the condenser, tobe cooled by the transport of heat energy from the ambient air flowingthrough the ambient air supply line to the bleed air flowing through thebleed air supply line. Preferably, the condenser is supplied with bleedair flowing through the bleed air supply line downstream of the bleedair turbine, wherein the term “downstream,” in this context, refers tothe direction of flow of bleed air through the bleed air supply line.This configuration ensures that the bleed air is supplied to thecondenser at the desired low temperature. Within the condenser, theambient air preferably is cooled below the dew point of water in orderto condense the free water present in the ambient air flowing throughthe ambient air supply line before the ambient air is directed into themixer of the aircraft air conditioning system.

In addition, a water separator may be arranged in the ambient air supplyline, in particular downstream of the condenser, wherein the term“downstream,” in this context, refers to the direction of flow ofambient air through the ambient air supply line. For example, the waterseparator may be designed in the form of a high pressure separator andmay serve to remove liquid water condensed from the ambient air streamupon being cooled in the condenser. Liquid water removed from theambient in the water separator may be discharged into the ram airchannel so as to be discharged to the ambient atmosphere, together withthe ram air flowing through the ram air channel. In a particularpreferred embodiment of the aircraft air conditioning system, waterremoved from the ambient air in the water separator, via a spring nozzleor another suitable supply device, is injected into the ram air channel,preferably upstream of an ambient air cooler and/or a precooler arrangedin the ram air channel, wherein the term “upstream,” in this context,refers to the direction of flow of ram air through the ram air channel.By injecting the liquid water into the ram air channel, the ram airflowing through the ram air channel may be cooled, thus enhancing thecooling capacity of the ram air prior to being supplied to the ambientair cooler and/or the precooler.

The aircraft air conditioning system may further comprise an ambient airturbine which is driven by the ambient air flowing through the ambientair supply line. In particular, the ambient air turbine is arranged inthe ambient air supply line downstream of the water separator, whereinthe term “downstream,” in this context, refers to the direction of flowof ambient air through the ambient air supply line. Within the ambientair turbine, the ambient air is expanded and thus cooled prior to beingsupplied to the mixer of the aircraft air conditioning system. Theambient air turbine may be coupled to the first compressor so as todrive the first compressor. For example, the ambient air turbine may bearranged with the first compressor and preferably also the bleed airturbine on a common shaft. This configuration allows a particularlyenergy-efficient operation of the aircraft air conditioning system.

A trim air line may branch off from the ambient air supply line, inparticular between the first compressor and the ambient air cooler. Thetrim air line may open into the ambient air supply line downstream ofthe ambient air turbine, wherein the term “downstream,” in this context,refers to the direction of flow of ambient air through the ambient airline. Via the trim air line, ambient air at an elevated pressure and atan elevated temperature may be supplied to the ambient air exiting theambient air turbine thus allowing ambient air at a desired temperatureand a desired pressure to be supplied to the mixer of the aircraft airconditioning system. An additional hot air tapping line may branch offfrom the trim air line. This allows to supply trim air also to otherareas than the mixer unit. A hot air regulation valve (HARV) may be usedto regulate the amount of trim air.

The aircraft air conditioning system may further comprise an altitudevent line which branches off from the ambient air supply line upstreamof the ambient air turbine and in particular upstream of the condenser.The altitude vent line may open into the ambient air supply linedownstream of the ambient air turbine, wherein the terms “upstream” and“downstream,” in this context, refer to the direction of flow of ambientair through the ambient air supply line. An altitude vent valve may bearranged in the altitude vent line so as to control the flow of ambientair through the altitude vent line. The altitude vent line thus bypassesthe condenser, the water separator and the ambient air turbine and may,during flight operation of an aircraft equipped with the aircraft airconditioning system, be used to increase permeability of the ambient airsupplied to the mixer of the aircraft air conditioning system and henceto reduce the effort for pressurizing the ambient air.

The aircraft air conditioning system may further be provided with anemergency ambient air supply line which directly connects the ambientair inlet to the mixer of the aircraft air conditioning system. Thus,via the emergency ambient air supply line, in the event of a failure ofthe aircraft air conditioning system, ambient air entering the systemvia the ambient air inlet can be directly supplied to the mixer of theaircraft air conditioning system, thus ensuring that at least the basicambient air demand of the aircraft air conditioning system is satisfied.A shut off valve (SOV) may be used to open this emergency air supplyline when needed.

The aircraft air conditioning system may also be equipped with a bypassline which is adapted to bypass the reheater. For example, the bypassline may connect a portion of the bleed air supply line extendingupstream of the reheater to a portion of the bleed air supply lineextending downstream of the reheater, so as to bypass the reheater. Theterms “upstream” and “downstream,” in this context, refer to thedirection of flow of the bleed air through the bleed air supply line. Abypass valve may be arranged in the bypass line and may be adapted tocontrol the flow of bleed air through the bypass line.

A further bypass line may be provided which is adapted to bypass theprecooler. For example, the further bypass line may connect a portion ofthe bleed air supply line extending between the reheater and theprecooler to a portion of the bleed air supply line extending downstreamof the precooler, so as to bypass the precooler. The terms “upstream”and “downstream,” in this context, again refer to the direction of flowof the bleed air through the bleed air supply line. A further bypassvalve may be arranged in the further bypass line and may be adapted tocontrol the flow of bleed air through the further bypass line.

Operation of the bypass valves and hence the flows of bleed air throughthe bypass line and the reheater, respectively, and the flows of bleedair through the further bypass line and the precooler, respectively, maybe controlled in dependence on a desired discharge temperature of thebleed air turbine so as to optimize the cooling performance of theaircraft air conditioning system. Furthermore, by supplying a sufficientamount of hot bleed air which bypasses the reheater and/or the precoolerto the bleed air turbine, icing of the bleed air turbine can beprevented.

A cabin exhaust air branch line may branch off from the cabin exhaustair line just before the air enters an ejector device and may open intothe ram air channel, preferably downstream of the ambient air precoolerand the precooler. The term “downstream,” in this context, refers to thedirection of flow of ram air through the ram air channel. A cabin airdump valve may be disposed in the cabin exhaust air branch line inparticular downstream of the cabin exhaust air turbine and serves tocontrol the flow of cabin exhaust air to be dumped either upstream ofthe ambient air precooler and the precooler or downstream of them. Incase the cabin exhaust air branch line opens into the ram air channeldownstream of the ambient air precooler and the precooler, undesiredheating of the ram air prior to being supplied to the ambient airprecooler and the precooler due to the supply of cabin exhaust air tothe ram air channel can be avoided.

The aircraft air conditioning system may further comprise a backup linewhich branches off from the bleed air supply line, in particulardownstream of the condenser. The term “downstream,” in this context,refers to the direction of flow of bleed air through the bleed airsupply line. A backup valve may be arranged in the backup line so as tocontrol the flow of bleed air through the backup line. For example, thebackup valve may be designed in the form of a three-way valve which maybe disposed in the region of a branch point between the backup line andthe bleed air supply line. Via the backup line, bleed air can besupplied to the mixer of the aircraft air conditioning system in theevent of a failure of the ambient air system, i.e., in the event thatthe supply of ambient air to the mixer is interrupted or affected inorder to satisfy the air demand of the mixer. However, during operationof the aircraft air conditioning system in pure ambient air mode, thebackup valve remains closed in order to prevent bleed air from enteringthe mixer. As an alternative, the backup valve may be used to controlthe supply of bleed air, either the total amount of bleed air or only apart of the bleed air to the mixer. This operation can be an intendednormal mode of operation or only a failure mode acting as backup to thepure ambient air mode.

The aircraft air conditioning system may further be equipped with ableed air branch line which branches off from the bleed air supply lineand which is adapted have hot (untreated) bleed air flowingtherethrough. For example, the bleed air branch line may branch off fromthe bleed air supply line upstream of the reheater and also upstream ofan upstream-side connecting point of the bypass line with the bleed airsupply line. The term “upstream,” in this context, refers to thedirection of flow of the bleed air through the bleed air supply line. Ahot air regulation valve may be disposed in the bleed air branch line soas to control the flow of hot bleed air through the bleed air branchline. In case the aircraft air conditioning system should be used forheating purposes, the hot air regulation valve can be suitablycontrolled in order to allow a desired volume flow of hot bleed airthrough the bleed air branch line which may be used for heating purposesin addition to or instead of hot ambient air flowing through theadditional trim air line. It should, however, be noted that the bleedair flowing through the bleed air branch line should not be directedinto the mixer so as to ensure that, at least during normal operation ofthe aircraft air conditioning system, the mixer is exclusively suppliedwith ambient air and, if desired, recirculation air which isrecirculated from the aircraft cabin to be air conditioned.

At least one of the bleed air supply line and the cabin exhaust air linemay open into the ram air channel. By discharging residual bleed air andresidual cabin exhaust air into the ram air channel, the flow of ram airthrough the ram air channel can be reduced while still providing asufficient amount of cooling energy to the ambient air cooler and/or theprecooler arranged in the ram air channel. In a particular preferredembodiment, the cabin exhaust air line is connected to an ejectordisposed in the ram air channel. The ejector preferably is adapted toinject the cabin exhaust air flowing through the cabin exhaust air lineat an increased speed and pressure thus enhancing the cooling capacityof the cabin exhaust air. The ejector may be driven by the bleed airexiting the bleed air supply line thus allowing a particularlyenergy-efficient operation of the ejector.

In a method of operating an aircraft air conditioning system a flow ofambient air is guided through an ambient air supply line having a firstend connected to an ambient air inlet and a second end connected to amixer of the aircraft air conditioning system so as to supply ambientair to the mixer. The ambient air flowing through the ambient air supplyline is compressed via a first compressor arranged in the ambient airsupply line. A flow of bleed air bled off from an engine or an auxiliarypower unit is guided through a bleed air supply line having a first endconnected to the engine or the auxiliary power unit and a second endconnected to an aircraft environment. A bleed air turbine is driven bythe bleed air flowing through the bleed air supply line, the bleed airturbine being coupled to the first compressor so as to drive the firstcompressor.

A flow of ambient air may be guided through an ambient air branch linebranching off from the ambient air supply line upstream of the firstcompressor and opening into the ambient air supply line downstream ofthe first compressor. The ambient air flowing through the ambient airbranch line may be compressed by means of a second compressor arrangedin the ambient air branch line. A flow of cabin exhaust air may beguided through a cabin exhaust air line. The flow of cabin exhaust airthrough the cabin exhaust air line in particular may be controlled via acabin exhaust air control valve arranged in the cabin exhaust air line.A cabin exhaust air turbine may be driven by means of the cabin exhaustair flowing through the cabin exhaust air line, the cabin exhaust airturbine being coupled to the second compressor so as to drive the secondcompressor.

A flow of ambient air may be guided through a connecting line connectingthe ambient air supply line downstream of the first compressor to theambient air branch line upstream of the second compressor. The supply ofambient air to the second compressor may be controlled via an ambientair supply control valve. In particular, the supply of ambient air tothe second compressor, via the ambient air supply control valve, may becontrolled by controlling at least one of the flow of ambient airthrough the connecting line and the flow of ambient air through theambient air branch line. The operation of the cabin exhaust air controlvalve and the ambient air supply control valve may be controlled in asynchronized manner in dependence on an ambient air demand of theaircraft air conditioning system.

A flow of compressed air may be guided through a compressed airrecirculation connecting a portion of the ambient air supply line whichextends downstream of the first compressor with a portion of the ambientair supply line which extends upstream of the first compressor. The flowof compressed air exiting the first compressor through the compressedair recirculation line back to the first compressor may be controlled bymeans of a recirculation control valve.

The cabin exhaust air flowing through the cabin exhaust air line may beheated via a reheater arranged in the cabin exhaust air line, inparticular upstream of the cabin exhaust air turbine, preferably bytransporting heat energy from the bleed air flowing through the bleedair supply line to the cabin exhaust air flowing through the cabinexhaust air line. The bleed air flowing through the bleed air supplyline may be cooled by means of a precooler arranged in the bleed airsupply line, preferably by transporting heat energy from the bleed airflowing through the bleed air supply line to ram air flowing through aram air channel. The reheater may be thermally coupled to the bleed airsupply line either upstream or downstream of the precooler.

The ambient air flowing through the ambient air supply line may becooled via an ambient air cooler arranged in the ambient air supplyline, in particular downstream of the first compressor, preferably bytransporting heat energy from the ambient air flowing through theambient air supply line to ram air flowing through the ram air channel.Water contained in the ambient air flowing through the ambient airsupply line may be condensed by means of a condenser arranged in theambient air supply line, in particular downstream of the ambient aircooler, preferably by transporting heat energy from the ambient airflowing through the ambient air supply line to the bleed air flowingthrough the bleed air supply line. Water contained in the ambient airflowing through the ambient air supply line may be separated from theambient air via a water separator arranged in the ambient air supplyline, in particular downstream of the condenser.

An ambient air turbine may be driven by the ambient air flowing throughthe ambient air supply line, the ambient air turbine preferably beingcoupled to the first compressor so as to drive the first compressor. Aflow of ambient air may be guided through a trim air line branching offfrom the ambient air supply line between the first compressor and theambient air cooler and opening into the ambient air supply linedownstream of the ambient air turbine. A flow of ambient air may beguided through an altitude vent line branching off from the ambient airsupply line upstream of the ambient air turbine, in particular upstreamof the condenser, and opening into the ambient air supply linedownstream of the ambient air turbine.

The flow of ambient air through the altitude vent line preferably iscontrolled by means of an altitude vent valve arranged in the altitudevent line. A flow of ambient air may be guided through an emergencyambient air supply line directly connecting the ambient air inlet to themixer of the aircraft air conditioning system. A flow of bleed air maybe guided through a bypass line so as to bypass the reheater. The flowof bleed air through the bypass line may be controlled via a bypassvalve. A flow of bleed air may be guided through a further bypass lineso as to bypass the precooler. The flow of bleed air through the furtherbypass line may be controlled by means of a further bypass valve. A flowof cabin exhaust air may be guided through a cabin exhaust air branchline branching off from the cabin exhaust air line, in particulardownstream of the cabin exhaust air turbine, and opening into the ramair channel, in particular downstream of the ambient air precooler andthe precooler. The flow of cabin exhaust air through the cabin exhaustair branch line, in particular downstream of the cabin exhaust airturbine, may be controlled via a cabin air dump valve.

A flow of bleed air may be guided through a backup line branching offfrom the bleed air supply line, in particular downstream of thecondenser. The flow of bleed air through the backup line preferably iscontrolled by means of a backup valve arranged in the backup line. Aflow of hot bleed air may be guided through a bleed air branch linebranching off from the bleed air supply line. The flow of hot bleed airthrough the bleed air branch line may be controlled via a hot airregulation valve.

At least one of bleed air flowing through the bleed air supply line andcabin exhaust air flowing through the cabin exhaust air line may beguided into the ram air channel. The cabin exhaust air preferably issupplied to the ram air channel via an ejector disposed in the ram airchannel which is adapted to inject the cabin exhaust air flowing throughthe cabin exhaust air line into the ram air channel. The ejector may bedriven by the bleed air exiting the bleed air supply line.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention now are described in greaterdetail with reference to the appended schematic drawings, wherein

FIG. 1 shows a schematic representation of a first embodiment of anaircraft air conditioning system,

FIG. 2 shows a schematic representation of a second embodiment of anaircraft air conditioning system,

FIG. 3 shows a schematic representation of a third embodiment of anaircraft air conditioning system,

FIG. 4 shows a schematic representation of a fourth embodiment of anaircraft air conditioning system,

FIG. 5 shows a schematic representation of a fifth embodiment of anaircraft air conditioning system, and

FIG. 6 shows a schematic representation of a sixth embodiment of anaircraft air conditioning system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aircraft air conditioning system 10 according to FIG. 1 comprises anambient air supply line 12. The ambient air supply line 12 has a firstend which is connected to an ambient air inlet 14. The ambient air inlet14, in the embodiment of an aircraft air conditioning system 10 shown inthe drawings, is designed in the form of a scoop inlet. A second end ofthe ambient air supply line 12 is connected to a mixer 16 of theaircraft air conditioning system 10. Within the mixer 16, ambient airsupplied to the mixer 16 via the ambient air supply line 12 is mixedwith recirculation air discharged from a cabin of an aircraft equippedwith the aircraft air conditioning system 10. The mixed air generatedwithin the mixer 16 finally is supplied to the aircraft cabin in orderto air-condition the aircraft cabin.

The ambient air entering the aircraft air conditioning system 10 via theambient air inlet 14 is supplied to a first compressor 18 which isarranged in the ambient air supply line 12. Within the first compressor18, the ambient air is compressed to an elevated pressure. Furthermore,the compressed air exiting the first compressor 18 has a temperaturewhich is higher than the temperature of the ambient air supplied to thefirst compressor 18.

An ambient air branch line 20 branches off from the ambient air supplyline 12 upstream of the first compressor 18 and opens into the ambientair supply line 12 downstream of the first compressor 18. A secondcompressor 22 is arranged in the ambient air branch line 20 which isadapted to compress the ambient air flowing through the ambient airbranch line 20. Thus, ambient air exiting the second compressor 22 has ahigher pressure and also a higher temperature than the ambient airsupplied to the second compressor 22.

A connecting line 24 connects the ambient air supply line 12 downstreamof the first compressor 18 to the ambient air branch line 20 upstream ofthe second compressor 22. Via the connecting line 24, the firstcompressor 18 and the second compressor 22 can be connected in series,thus allowing compressed ambient air exiting the first compressor 18 tobe further compressed by means of the second compressor 22. An ambientair supply control valve 26 is arranged in the region of an opening ofthe connecting line 24 into the ambient air branch line 20 and isadapted to control the supply of ambient air to the second compressor22. In particular, the ambient air supply valve 26 is designed in theform of a three-way valve which allows to either interrupt or to allowthe supply of ambient air to the second compressor 22 and, furthermore,to connect the first compressor 18 and the second compressor 22 eitherin parallel or in series.

Compressed ambient air exiting the first and the second compressor 18,22, respectively, is supplied to an ambient air cooler 28 which isarranged in the ambient air supply line 12 downstream of the firstcompressor 18. The ambient air cooler 28 is disposed in a ram airchannel 30 and thus is thermally coupled to the ram air channel 30.During flight operation of an aircraft equipped with the aircraft airconditioning system 10, ram air flows through the ram air channel 30,entering the ram air channel 30 via a ram air inlet 31. During groundoperation of the aircraft, ambient air flows through the ram air channel30, conveyed through the ram air channel 30 by means of a blower 33.Alternatively, instead of the blower 33, a jet pump may be used toconvey ambient air through the ram air channel 30 during groundoperation of the aircraft. The ram air flowing through the ram airchannel 30 exits the ram air channel 30 via a ram air outlet. A ram airoutlet flap (not shown) may be used to control the ram pressure in theram air channel 30.

The ambient air flowing through the ambient air supply line 12 thus, inthe ambient air cooler 28, is cooled by transporting heat energy fromthe ambient air to the ram air flowing through the ram air channel 30.In the specific embodiment of an aircraft air conditioning system 10according to FIG. 1, the ambient air cooler 28 is disposed in a ram airchannel branch 30 a and thus cooled by the ram air flowing through theram air channel branch 30 a.

In the embodiments of an aircraft air conditioning system 10 which aredepicted in the drawings, the ambient air inlet 14 and the ram air inlet31 are designed as separate devices. It is, however, also conceivable todesign the ambient air inlet 14 and the ram air inlet 31 in the form ofa combined air intake, in particular in the form of a combinedNACA/Scoop air intake which may be equipped with a deflection flap. Thesize of the combined air intake may be adjusted to the operatingconditions of the combined air intake during flight operation of anaircraft equipped with the aircraft air conditioning system 10.Furthermore, the combined air intake may comprise a ground inlet flapfor suitably controlling the supply of air via the combined air intakeduring ground operation of the aircraft.

Ambient air exiting the ambient air cooler 28 further is directed to acondenser 32 which is disposed in the ambient air supply line 12downstream of the ambient air cooler 28. Within the condenser 32, theambient air is cooled below the dew point of water in order to transferfree water contained in the ambient air flowing through the ambient airsupply line 12 into the liquid state of aggregation. The liquid watercondensed within the condenser 32 is extracted from the ambient airstream flowing through the ambient air supply line 12 via a waterseparator 34 which is disposed in the ambient air supply line 12downstream of the condenser 32.

The water extracted from the ambient air stream by means of the waterseparator 34, via a water supply line 36, is directed to the ram airchannel 30. In particular, the water is injected into the ram airchannel 30 via an injector nozzle 38 disposed in the ram air channel 30.By the injection of water, the ram air flowing through the ram airchannel 30 can be cooled thus enhancing the cooling capacity of the ramair channel 30. As a result, the ram air flow through the ram airchannel 30 and thus the aerodynamic drag caused by the ram air channel30 can be reduced without affecting the cooling capacity of the ram airchannel 30.

Finally, prior to being supplied to the mixer 16, the ambient airflowing through the ambient air supply line 12 is supplied to an ambientair turbine 40 which is arranged in the ambient air supply line 12downstream of the water separator 34. Within the ambient air turbine 40,the ambient air is expanded and thus cooled. In order to prevent theambient air turbine 40 from being damaged by remaining water droplets inthe flow of ambient air downstream of the water separator 34, theambient air turbine 40 may be of a robust design which is not sensitiveto water droplets. Alternatively or additionally thereto, an additionalreheater (not shown) may be provided in the ambient air supply line 12upstream of the ambient air turbine 40 which serves to evaporate anyremaining water droplets in the flow of ambient air before the flow ofambient air is supplied to the ambient air turbine 40.

The aircraft air conditioning system 10 further comprises a trim airline 42 which branches off from the ambient air supply line 12 betweenthe first compressor 18 and the ambient air cooler 28 and which opensinto the ambient air supply line 12 downstream of the ambient airturbine 40. The flow of trim air through the trim air line 42 iscontrolled via a trim air valve 44 arranged in the trim air line 42. Viathe trim air line 42, ambient air exiting the first and the secondcompressor 18, 22, respectively, and thus having an elevated pressureand an elevated temperature can be supplied to the ambient air flowingthrough the ambient air supply line 12 downstream of the ambient airturbine 40 and thus having a lower pressure and a lower temperature soas to adjust the pressure and the temperature of the ambient air flowingthrough the ambient air supply line 12 as desired before the ambient airis supplied to the mixer 16. In case the aircraft air conditioningsystem 10 should be used for heating purposes, the trim air valve 44 canbe suitably controlled in order to allow a higher volume flow of hotambient air exiting the first and the second compressor 18, 22,respectively, to be directed into the mixer 16 via the trim air line 42.

An altitude vent line 46 branches off from the ambient air supply line12 upstream of the condenser 32 and hence also upstream of the ambientair turbine 40 and opens into the ambient air supply line 12 downstreamof the ambient air turbine 40. The altitude vent line 46 thus bypassesthe condenser 32, the water separator 34 and the ambient air turbine 40.The flow of ambient air through the altitude vent line 46 is controlledby means of an altitude vent valve 48 which is arranged in the altitudevent line 46. By guiding ambient air through the altitude vent line 46so as to bypass the condenser 32, the water separator 34 and the ambientair turbine 40, the permeability of ambient air through the ambient airsupply line 12 can be increased and thus the effort for pressurizing theambient air in the first and the second compressor 18, 22, respectively,can be reduced. This is particularly advantageous when an aircraftequipped with the aircraft air conditioning system 10 is operated atcruising altitude at a low ambient pressure.

In case the aircraft air conditioning system 10 should be used forheating purposes, the first and/or the second cabin air compressor 18,22 typically is/are operated in such a manner that the ambient airflowing through the ambient air supply line 12 is compressed to apressure which exceeds the pressure in the aircraft cabin supplied withconditioned air by the aircraft air conditioning system 10. The altitudevent valve 48 and/or the trim air valve 44 then may be used asthrottling devices for reducing the pressure of the ambient air flowingthrough the ambient air supply line 12 before being supplied to themixer 16.

Finally, an emergency ambient air supply line 50 directly connects theambient air inlet 14 to the mixer 16 of the aircraft air conditioningsystem 10. The supply of ambient air to the emergency ambient air supplyline 50 may be controlled via a suitable valve (not shown in thedrawings). In the event of a failure of the aircraft air conditioningsystem 10, ambient air can be supplied to the mixer 16 via the emergencyambient air supply line 50 in order to satisfy at least a basic ambientair demand of the mixer 16.

The aircraft air conditioning system 10 further comprises a bleed airsupply line 52 which allows a flow of bleed air bled off from an engine54 of an aircraft equipped with the aircraft air conditioning system 10therethrough. A first end of the bleed air supply line 52 is connectedto the engine 54. Alternatively, the bleed air supply line 52 can beconnected to an auxiliary power unit of the aircraft so as to besupplied with bleed air from the auxiliary power unit. A bleed aircontrol valve 56, which is arranged in the bleed air supply line 52,serves to control the flow of bleed air through the bleed air supplyline 52. A second end of the bleed air supply line 52 is connected tothe aircraft environment, i.e., the ambient atmosphere so as todischarge the bleed air flowing through the bleed air supply line 52over board. Specifically, the second end of the bleed air supply line 52is connected to the ram air channel 30 such that the bleed air exitingthe bleed air supply line 52 is discharged to the aircraft environmentvia the ram air channel 30.

The bleed air supplied to the aircraft air conditioning system 10 viathe bleed air supply line 52 first is supplied to a precooler 58 whichis disposed in the bleed air supply line 52 and which is thermallycoupled to the ram air channel 30. Thus, the bleed air flowing throughthe bleed air supply line 52, in the precooler 58, is cooled by thetransport of heat energy from the bleed air flowing through the bleedair supply line 52 to the ram air flowing through the ram air channel30. In particular, in the arrangement shown in FIG. 1, the precooler 58is disposed in a second ram air channel branch 30 b arranged in parallelto the first ram air channel branch 30 a. Thus, the ambient air cooler28 and the precooler 58 are arranged in the ram air channel 30 inparallel allowing the ambient air cooler 28 and the precooler 58 to becooled by ram air flowing through the ram air channel 30 independentfrom each other, i.e., with ram air having substantially the sametemperature.

Precooled bleed air exiting the precooler 58, via the bleed air supplyline 52, is directed to a bleed air turbine 60 so as to drive the bleedair turbine 60 and to be thereby expanded and cooled. The bleed airturbine 60 is coupled to the first compressor 18. In particular, thebleed air turbine 60 is arranged with the first compressor 18 and theambient air turbine 40 on a common shaft. In the aircraft airconditioning system 10, the driving energy for driving the firstcompressor 18 thus is provided by the bleed air turbine 60 driven by thebleed air flowing through the bleed air supply line 52. An optimum useof the energy contained in the ambient air flowing the ambient airsupply line 12 is made by coupling the first compressor 18 also to theambient air turbine 40 in order to provide additional driving energy tothe first compressor 18.

The bleed air exiting the bleed air turbine 60, via the bleed air supplyline 52, is directed to the condenser 32. Thus, the condenser 32 isthermally coupled to the bleed air supply line 52 and the ambient airflowing through the ambient air supply line 12, in the condenser 32, iscooled by the transport of heat energy from the ambient air to the bleedair flowing through the bleed air supply line 52. The bleed air exitingthe condenser 32 is discharged into the ram air channel 30 as describedabove.

A backup line 62 branches off from the bleed air supply line 52downstream of the condenser 32. The backup line opens into the ambientair supply line 12 downstream of the ambient air turbine 40. The flow ofbleed air through the backup line 62 is controlled by means of a backupvalve 64. The backup valve 64 is designed in the form of a three-wayvalve which is arranged in the region of a branch point between thebleed air supply line 52 and the backup line 62. Via the backup line 62,in the event of a failure of the aircraft air conditioning system 10,bleed air can be directed to the mixer 16 in order to satisfy a basicair demand of the mixer 16. However, during normal operation of theaircraft air conditioning system 10, the backup valve 64 is closed so asto ensure that the mixer 16 is exclusively supplied with ambient airand, if desired, recirculation air which is recirculated from theaircraft cabin to be air conditioned.

The aircraft air conditioning system 10 further comprises a cabinexhaust air line 66 allowing a flow of cabin exhaust air there through.In particular, the cabin exhaust air line 66 is connected to theaircraft cabin and serves to discharge cabin exhaust air from theaircraft cabin, wherein the flow of cabin exhaust air through the cabinexhaust air line 66 is controlled via a cabin exhaust air control valve68. After being discharged from the aircraft cabin, the cabin exhaustair first is directed through a reheater 70 which is arranged in thecabin exhaust air line 66. Within the reheater 70, the cabin exhaust airflowing through the cabin exhaust air line 66 is heated to an elevatedtemperature. The reheater 70 is thermally coupled to the bleed airsupply line 52 downstream of the precooler 58. Thus, the cabin exhaustair flowing through the cabin exhaust air line 66, in the reheater 70,is heated by the transport of heat energy from the bleed air flowingthrough the bleed air supply line 52 to the cabin exhaust air flowingthrough the cabin exhaust air line 66.

The cabin exhaust air exiting the reheater 70 is directed to a cabinexhaust air turbine 72 so as to drive the cabin exhaust air turbine 72and to thereby be expanded and cooled. The cabin exhaust air turbine 72driven by the cabin exhaust air flowing through the cabin exhaust airline 66 is coupled to the second compressor 22 so as to drive the secondcompressor 22. In particular, the second compressor 22 and the cabinexhaust air turbine 72 are disposed on a common shaft. Thus, the energycontained in the cabin exhaust air is used to drive the secondcompressor 22 in order to provide the mixer 16 with a sufficient amountof compressed ambient air also when the aircraft equipped with theaircraft air conditioning system 10 is operated at high altitude, i.e.,at a low ambient pressure.

An electronic control unit 74 of the aircraft air conditioning system 10is adapted to control the operation of the cabin exhaust air controlvalve 68 and the ambient air supply control valve 26. In particular, theelectronic control unit 74 controls the operation of the cabin exhaustair control valve 68 and the operation of the ambient air supply controlvalve 26 in a synchronized manner in dependence on an ambient air demandof the aircraft air conditioning system 10. In particular, the cabinexhaust air control valve 68 and the ambient air supply control valve26, under the control of the electronic control unit 74, are opened in asynchronized manner so as to increase the supply of cabin exhaust air tothe cabin exhaust air turbine 72 and to also increase the supply ofambient air to the second compressor 22 in order to satisfy an increasedambient air demand of the aircraft air conditioning system 10. To thecontrary, the cabin exhaust air control valve 68 and the ambient airsupply control valve 26, under the control of the electronic controlunit 74, are closed so as to reduce the supply of cabin exhaust air tothe cabin exhaust air turbine 72 and the supply of ambient air to thesecond compressor 22 in response to a reduced ambient air demand of theaircraft air conditioning system 10.

Furthermore, as already indicated above, by suitably controlling theambient air supply control valve 26 by means of the electronic controlunit 74, the first compressor 18 and the second compressor 22 may beoperated in parallel or in series, as desired. Parallel operation of thefirst compressor 18 and the second compressor 22 may lead to a reducedram air consumption of the aircraft air conditioning system 10, buttypically leads to a slightly higher bleed air demand.

Cabin exhaust exiting the cabin exhaust air turbine 72 is dischargedinto the ram air channel 30. In particular, the cabin exhaust air line66 is connected to an ejector 76 which is disposed in the ram airchannel 30 and which is adapted to inject the cabin exhaust air flowingthrough the cabin exhaust air line 66 into the ram air channel 30. Byinjecting cool cabin exhaust air into the ram air channel 30, thecooling capacity of the ram air channel 30 can be increased. The ejector76 is driven by the bleed air exiting the bleed air supply line 52, thusallowing a particularly energy efficient operation of the ejector 76.

The aircraft air conditioning system 10 depicted in FIG. 2 differs fromthe arrangement according to FIG. 1 only in that the reheater 70 isthermally coupled to the bleed air supply line 52 upstream of theprecooler 58. While by arranging the reheater upstream of the precooler,the bleed air consumption of the aircraft air conditioning system 10 canbe reduced, an arrangement of the reheater 70 downstream of theprecooler 58 as shown in FIG. 1 allows a decrease of the ram air flowthrough the ram air channel 30. Otherwise, the structure and thefunction of the aircraft air conditioning system 10 according to FIG. 2correspond to the structure and the function of the arrangement depictedin FIG. 1.

The aircraft air conditioning system 10 according to FIG. 3 differs fromthe configuration according to FIG. 2 in that the ram air channel 30 nolonger comprises a first ram air channel branch 30 a and a second ramair channel branch 30 b. Instead, the ambient air cooler 28 and theprecooler 58 are arranged in the ram air channel 30 in series. Inparticular, the ambient air cooler 28 is arranged in the ram air channel30 upstream of the precooler 58. In the configuration according to FIG.3, the precooler 58 thus is supplied with ram air which is alreadyheated by the transfer of heat from the ambient air flowing through theambient air supply line 12 in the ambient air cooler 28. However, theram air channel 30, in the aircraft air conditioning system 10 accordingto FIG. 3, is of a particularly simple and thus lightweight design.Otherwise, the structure and the function of the aircraft airconditioning system 10 according to FIG. 3 correspond to the structureand the function of the arrangement depicted in FIG. 2.

The aircraft air conditioning system 10 according to FIG. 4 differs fromthe configuration according to FIG. 3 in that a shut off valve 78 isinstalled in the emergency ambient air supply line 50. If need be, theshut off valve 78 is opened to allow air flowing into the mixer 16.Furthermore, an additional trim air line 80 branches off from the trimair line 42 upstream of the trim air valve 44. The additional trim airline 80 can be used to supply trim air to other areas than the mixer 16.A trim air regulation valve 82 installed in the additional trim air line80 is used to modulate this trim air flow. Otherwise, the structure andthe function of the aircraft air conditioning system 10 according toFIG. 4 correspond to the structure and the function of the arrangementdepicted in FIG. 3.

The aircraft air conditioning system 10 according to FIG. 5 differs fromthe configuration according to FIG. 3 in that, like in the arrangementof FIG. 4, a shut off valve 78 is installed in the emergency ambient airsupply line 50. Furthermore, like the arrangement of FIG. 4, also theaircraft air conditioning system 10 according to FIG. 5 comprises anadditional trim air line 80 which branches off from the trim air line 42upstream of the trim air valve 44 and a trim air regulation valve 82which is installed in the additional trim air line 80 for modulating thetrim air flow through the additional trim air line.

The aircraft air conditioning system 10 according to FIG. 5 furthercomprises a compressed air recirculation line 84 which branches off fromthe ambient air supply line 12 downstream of the first compressor 18 andwhich opens back into the ambient air supply line 12 upstream of thefirst compressor 18. Thus, compressed air exiting the first compressor18, via the compressed air recirculation line 84, may be recirculated tothe first compressor 18 to be compressed again. This allows animprovement of the heating performance of the aircraft air conditioningsystem 10 in case the aircraft air conditioning system 10 should be usedfor heating purposes. The flow of compressed air exiting the firstcompressor 18 through the compressed air recirculation line 84 back tothe first compressor 18 is controlled via a recirculation control valve86.

The aircraft air conditioning system 10 according to FIG. 5 also isequipped with a bypass line 87 which connects a portion of the bleed airsupply line 52 extending upstream of the reheater 70 to a portion of thebleed air supply line 52 extending downstream of the reheater 70, so asto bypass the reheater 70. A bypass valve 88 is arranged in the bypassline 87 so as to control the flow of bleed air through the bypass line87. A further bypass line 89 connects a portion of the bleed air supplyline 52 extending between the reheater 70 and the precooler 58 to aportion of the bleed air supply line 52 extending downstream of theprecooler 58, so as to bypass the precooler 58. A further bypass valve90 is arranged in the further bypass line 89 so as to control the flowof bleed air through the further bypass line 89.

Operation of the bypass valves 88, 90 and hence the flows of bleed airthrough the bypass line 87 and the reheater 70, respectively, and theflows of bleed air through the further bypass line 89 and the precooler58, respectively, may be controlled in dependence on a desired dischargetemperature of the bleed air turbine 60 so as to optimize the coolingperformance of the aircraft air conditioning system 10. Furthermore, bysupplying a sufficient amount of hot bleed air which bypasses thereheater 70 and/or the precooler 58 to the bleed air turbine 60, icingof the bleed air turbine 60 can be prevented.

The aircraft air conditioning system 10 of FIG. 5, further comprises anelectric motor 92 which is associated with the compressor/turbinearrangement defined by the second compressor 22 and the cabin exhaustair turbine 72. Under certain operating conditions of the aircraft airconditioning system 10, e.g., during ground or low altitude operation ofan aircraft equipped with the aircraft air conditioning system 10 underhumid ambient conditions, the discharge temperatures of the bleed airturbine 60 should not fall below 0° C. in order to avoid icing of thebleed air turbine 60. In case of a high cooling energy demand, thecompressor/turbine arrangement defined by the second compressor 22 andthe cabin exhaust air turbine 72 then has to provide for a sufficientcooling performance of the aircraft air conditioning system 10, whereinthe electric motor 92, if need be, may boost the driving performance ofthe cabin exhaust air turbine 72 or may also be used as the only drivingmeans for driving the second compressor 22. In addition, thecompressor/turbine arrangement defined by the second compressor 22 andthe cabin exhaust air turbine 72, when being provided with an additionalelectric motor 92 for boosting the driving performance of the cabinexhaust air turbine 72 may be used for driving a jet pump which isarranged in the ram air channel 30 for conveying ambient air through theram air channel 30 during ground operation of the aircraft.

The electric motor 92 is coupled to the shaft coupling the secondcompressor 22 to the cabin exhaust air turbine 72 via a clutch (notshown). This allows the electric motor 92 to be decoupled from the shaftwhen the electric motor 92 is not in operation. In addition, also thecabin exhaust air turbine 72 is coupled to the shaft via a clutch (notshown) so as to allow a decoupling of the cabin exhaust air turbine 72from the shaft when only the electric motor 92 operated for driving thesecond compressor 22.

In the aircraft air conditioning system 10 of FIG. 5, a heat exchanger94 is disposed in the connecting line 24 connecting the outlet of thefirst compressor 18 to the inlet of the second compressor 22. The heatexchanger 94 acts as an intercooler and serves to cool the ambient airexiting the first compressor 18 before being supplied to the secondcompressor 22. As a result, the compression effort may be reduced.Moreover, the aircraft air conditioning system 10 comprises an ozoneconverter 96 which is disposed in the ambient air supply line 12downstream of the first and the second compressor 18, 22.

The ram air channel 30 of the aircraft air conditioning system 10depicted in FIG. 5 is equipped with a ram air check valve 97 whichallows the ram air flowing through the ram air channel 30 to bypass theblower 33 during flight operation of an aircraft equipped with theaircraft air conditioning system 10 when a sufficient amount of ram airis available and operation of the blower 33 is not needed.Alternatively, in case a ram air check valve 97 should be omitted, theblower 33 may be provided with variable fan blades.

A cabin exhaust air branch line 98 branches off from the cabin exhaustair line 66 just before the air enters the ejector 76 and opens into theram air channel 30 downstream of the ambient air precooler 28 and theprecooler 58. A cabin air dump valve 100 is disposed in the cabinexhaust air branch line 98 downstream of the cabin exhaust air turbine72 and serves to control the flow of cabin exhaust air to be dumpedeither upstream of the ambient air precooler 28 and the precooler 58 ordownstream of them. Since the cabin exhaust air branch line 98 opensinto the ram air channel 30 downstream of the ambient air precooler 28and the precooler 58, undesired heating of the ram air prior to beingsupplied to the ambient air precooler 28 and the precooler 58 due to thesupply of cabin exhaust air to the ram air channel 30 can be avoided.

Finally, the aircraft air conditioning system 10 of FIG. 5 is equippedwith a bleed air branch line 102 which branches off from the bleed airsupply line 52 upstream of the reheater 70 and also upstream of anupstream-side connecting point of the bypass line 87 with the bleed airsupply line 52. Hot (untreated) bleed air thus flows through the bleedair branch line 102. A hot air regulation valve 104 is disposed in thebleed air branch line 102 and serves to control the flow of hot bleedair through the bleed air branch line 102. In case the aircraft airconditioning system 10 should be used for heating purposes, the hot airregulation valve 104 can be suitably controlled in order to allow adesired volume flow of hot bleed air to be used for heating purposes,wherein the bleed air may be used in addition to or instead of hotambient air flowing through the additional trim air line 80.

It should, however, be noted that the bleed air flowing through thebleed air branch line 100 is not directed into the mixer 16, thusensuring that, at least during normal operation of the aircraft airconditioning system 10, the mixer 16 is exclusively supplied withambient air and, if desired, recirculation air which is recirculatedfrom the aircraft cabin to be air conditioned. Otherwise, the structureand the function of the aircraft air conditioning system 10 according toFIG. 4 correspond to the structure and the function of the arrangementdepicted in FIG. 3.

The aircraft air conditioning system 10 according to FIG. 6 differs fromthe configuration according to FIG. 5 in that, instead of the electricmotor 92, a further bleed air driven turbine 106 is used boost the cabinexhaust air turbine 72 when needed. The further bleed air driven turbine106 is supplied with bleed air via a bleed air branch line 108 branchingoff from the bleed air supply line 52 upstream of the bleed air turbine60. The flow of bleed air through the bleed air branch line 108 iscontrolled by means of a valve 110. The expanded bleed air is dischargedinto the ram air channel via a discharge line 112 and the ejector 76.Otherwise, the structure and the function of the aircraft airconditioning system 10 according to FIG. 6 correspond to the structureand the function of the arrangement depicted in FIG. 5.

Although specific features of the aircraft air conditioning system 10are described herein with reference to specific embodiments, thesefeatures may be combines as desired. For example, also the aircraft airconditioning system 10 of FIG. 1 may be equipped with an additional trimair line 80 or the aircraft air conditioning system 10 of FIG. 5 may beprovided with a reheater 70 which is thermally coupled to the bleed airsupply line 52 downstream of the precooler 58.

While at least one exemplary embodiment of the present invention(s) isdisclosed herein, it should be understood that modifications,substitutions and alternatives may be apparent to one of ordinary skillin the art and can be made without departing from the scope of thisdisclosure. This disclosure is intended to cover any adaptations orvariations of the exemplary embodiment(s). In addition, in thisdisclosure, the terms “comprise” or “comprising” do not exclude otherelements or steps, the terms “a” or “one” do not exclude a pluralnumber, and the term “or” means either or both. Furthermore,characteristics or steps which have been described may also be used incombination with other characteristics or steps and in any order unlessthe disclosure or context suggests otherwise. This disclosure herebyincorporates by reference the complete disclosure of any patent orapplication from which it claims benefit or priority.

The invention claimed is:
 1. An aircraft air conditioning systemcomprising: an ambient air supply line having a first end connected toan ambient air inlet and a second end connected to a mixer of theaircraft air conditioning system so as to supply ambient air to themixer, a first compressor arranged in the ambient air supply line andbeing configured to compress ambient air flowing through the ambient airsupply line, a bleed air supply line having a first end connected to anengine or an auxiliary power unit and a second end connected to anaircraft environment, a bleed air turbine driven by bleed air flowingthrough the bleed air supply line and being coupled to the firstcompressor so as to drive the first compressor, an ambient air branchline branching off from the ambient air supply line upstream of thefirst compressor and opening into the ambient air supply line downstreamof the first compressor, a second compressor arranged in the ambient airbranch line and being configured to compress the ambient air flowingthrough the ambient air branch line, a cabin exhaust air line allowing aflow of cabin exhaust air therethrough, and a cabin exhaust air turbinedriven by the cabin exhaust air flowing through the cabin exhaust airline and being coupled to the second compressor so as to drive thesecond compressor.
 2. The aircraft air conditioning system according toclaim 1, wherein a cabin exhaust air control valve is arranged in thecabin exhaust air line so as to control the flow of cabin exhaust airthrough the cabin exhaust air line.
 3. The aircraft air conditioningsystem according to claim 2, further comprising at least one of: aconnecting line connecting the ambient air supply line downstream of thefirst compressor to the ambient air branch line upstream of the secondcompressor, an ambient air supply control valve configured to control asupply of ambient air to the second compressor, wherein the ambient airsupply control valve is configured to control at least one of a flow ofambient air through the connecting line and a flow of ambient airthrough the ambient air branch line, a compressed air recirculation lineconnecting a portion of the ambient air supply line which extendsdownstream of the first compressor with a portion of the ambient airsupply line which extends upstream of the first compressor, and arecirculation control valve configured to control a flow of compressedair exiting the first compressor through the compressed airrecirculation line back to the first compressor.
 4. The aircraft airconditioning system according to claim 2, further comprising a reheaterarranged in the cabin exhaust air line, upstream of the cabin exhaustair turbine, and being thermally coupled to the bleed air supply line soas to transport heat energy from the bleed air flowing through the bleedair supply line to the cabin exhaust air flowing through the cabinexhaust air line.
 5. The aircraft air conditioning system according toclaim 4, wherein the reheater is thermally coupled to the bleed airsupply line either upstream or downstream of a precooler arranged in thebleed air supply line, the precooler being thermally coupled to a ramair channel so as to transport heat energy from the bleed air flowingthrough the bleed air supply line to ram air flowing through the ram airchannel.
 6. The aircraft air conditioning system according to claim 5,further comprising at least one of: an ambient air cooler arranged inthe ambient air supply line, downstream of the first compressor, andbeing thermally coupled to the ram air channel so as to transport heatenergy from ambient air flowing through the ambient air supply line toram air flowing through the ram air channel, a condenser arranged in theambient air supply line, downstream of the ambient air cooler, and beingthermally coupled to the bleed air supply line so as to transport heatenergy from ambient air flowing through the ambient air supply line tobleed air flowing through the bleed air supply line, a water separatorarranged in the ambient air supply line, downstream of the condenser, anambient air turbine driven by ambient air flowing through the ambientair supply line and being coupled to the first compressor so as to drivethe first compressor, a trim air line branching off from the ambient airsupply line between the first compressor and the ambient air cooler andopening into the ambient air supply line downstream of the ambient airturbine, an altitude vent line branching off from the ambient air supplyline upstream of the ambient air turbine, upstream of the condenser, andopening into the ambient air supply line downstream of the ambient airturbine, an altitude vent valve being arranged in the altitude vent lineso as to control a flow of ambient air through the altitude vent line,an emergency ambient air supply line directly connecting the ambient airinlet to the mixer of the aircraft air conditioning system, a bypassline configured to bypass the reheater, a bypass valve configured tocontrol a flow of bleed air through the bypass line, a further bypassline configured to bypass the precooler, a further bypass valveconfigured to control the flow of bleed air through the further bypassline, a cabin exhaust air branch line branching off from the cabinexhaust air line, downstream of the cabin exhaust air turbine, andopening into the ram air channel, downstream of the ambient air coolerand the precooler, and a cabin air dump valve configured to control theflow of cabin exhaust air, downstream of the cabin exhaust air turbine,through the cabin exhaust air branch line.
 7. The aircraft airconditioning system according to claim 6, further comprising at leastone of: a backup line branching off from the bleed air supply line,downstream of the condenser, and opening into the ambient air supplyline, downstream of the ambient air turbine, a backup valve beingarranged in the backup line so as to control a flow of bleed air throughthe backup line, and a bleed air branch line branching off from thebleed air supply line and configured to have hot bleed air flowingtherethrough, wherein a hot air regulation valve is configured tocontrol a flow of hot bleed air through the bleed air branch line. 8.The aircraft air conditioning system according to claim 5, wherein atleast one of the bleed air supply line and the cabin exhaust air lineopens into the ram air channel, wherein the cabin exhaust air line isconnected to an ejector disposed in the ram air channel configured toinject cabin exhaust air flowing through the cabin exhaust air line intothe ram air channel and which is driven by bleed air exiting the bleedair supply line.
 9. A method of operating an aircraft air conditioningsystem, the method comprising the steps: guiding a flow of ambient airthrough an ambient air supply line having a first end connected to anambient air inlet and a second end connected to a mixer of the aircraftair conditioning system so as to supply ambient air to the mixer, theflow of ambient air being controlled by a shut off valve, compressingthe ambient air flowing through the ambient air supply line via a firstcompressor arranged in the ambient air supply line, guiding a flow ofbleed air bled off from an engine or an auxiliary power unit through ableed air supply line having a first end connected to the engine or theauxiliary power unit and a second end connected to an aircraftenvironment, driving a bleed air turbine by the bleed air flowingthrough the bleed air supply line, the bleed air turbine being coupledto the first compressor so as to drive the first compressor, guiding aflow of ambient air through an ambient air branch line branching offfrom the ambient air supply line upstream of the first compressor andopening into the ambient air supply line downstream of the firstcompressor, compressing the ambient air flowing through the ambient airbranch line via a second compressor arranged in the ambient air branchline, guiding a flow of cabin exhaust air through a cabin exhaust airline, and driving a cabin exhaust air turbine via the cabin exhaust airflowing through the cabin exhaust air line, the cabin exhaust airturbine being coupled to the second compressor so as to drive the secondcompressor.
 10. The method according to claim 9, wherein the flow ofcabin exhaust air through the cabin exhaust air line being controlledvia a cabin exhaust air control valve arranged in the cabin exhaust airline.
 11. The method according to claim 10, further comprising at leastone of the steps of: guiding a flow of ambient air through a connectingline connecting the ambient air supply line downstream of the firstcompressor to the ambient air branch line upstream of the secondcompressor, controlling a supply of ambient air to the second compressorvia an ambient air supply control valve, by controlling, via the ambientair supply control valve, at least one of the flow of ambient airthrough the connecting line and the flow of ambient air through theambient air branch line, controlling an operation of the cabin exhaustair control valve and the ambient air supply control valve in asynchronized manner in dependence on an ambient air demand of theaircraft air conditioning system, guiding a flow of compressed airthrough a compressed air recirculation line connecting a portion of theambient air supply line which extends downstream of the first compressorwith a portion of the ambient air supply line which extends upstream ofthe first compressor, and controlling a flow of compressed air exitingthe first compressor through the compressed air recirculation line backto the first compressor via a recirculation control valve.
 12. Themethod according to claim 10, further comprising at least one of thesteps of: heating cabin exhaust air flowing through the cabin exhaustair line via a reheater arranged in the cabin exhaust air line, upstreamof the cabin exhaust air turbine, by transporting heat energy from bleedair flowing through the bleed air supply line to the cabin exhaust airflowing through the cabin exhaust air line, or cooling bleed air flowingthrough the bleed air supply line via a precooler arranged in the bleedair supply line, by transporting heat energy from bleed air flowingthrough the bleed air supply line to ram air flowing through a ram airchannel, wherein the reheater is thermally coupled to the bleed airsupply line either upstream or downstream of the precooler.
 13. Themethod according to claim 12, further comprising at least one of thesteps of: cooling ambient air flowing through the ambient air supplyline via an ambient air cooler arranged in the ambient air supply line,downstream of the first compressor, by transporting heat energy fromambient air flowing through the ambient air supply line to ram airflowing through the ram air channel, condensing water contained inambient air flowing through the ambient air supply line via a condenserarranged in the ambient air supply line, downstream of the ambient aircooler, by transporting heat energy from ambient air flowing through theambient air supply line to bleed air flowing through the bleed airsupply line, separating water contained in the ambient air flowingthrough the ambient air supply line via a water separator arranged inthe ambient air supply line, downstream of the condenser, driving anambient air turbine by ambient air flowing through the ambient airsupply line, the ambient air turbine being coupled to the firstcompressor so as to drive the first compressor, guiding a flow ofambient air through a trim air line branching off from the ambient airsupply line between the first compressor and the ambient air cooler andopening into the ambient air supply line downstream of the ambient airturbine, guiding a flow of ambient air through an altitude vent linebranching off from the ambient air supply line upstream of the ambientair turbine, upstream of the condenser, and opening into the ambient airsupply line downstream of the ambient air turbine, wherein the flow ofambient air through the altitude vent line is controlled via an altitudevent valve arranged in the altitude vent line, guiding a flow of ambientair through an emergency ambient air supply line directly connecting theambient air inlet to the mixer of the aircraft air conditioning system,guiding a flow of bleed air through a bypass line to bypass thereheater, controlling the flow of bleed air through the bypass line viaa bypass valve, guiding a flow of bleed air through a further bypassline to bypass the precooler, controlling the flow of bleed air throughthe further bypass line via a further bypass valve, guiding a flow ofcabin exhaust air through a cabin exhaust air branch line branching offfrom the cabin exhaust air line, downstream of the cabin exhaust airturbine, and opening into the ram air channel, downstream of the ambientair precooler and the precooler, and controlling the flow of cabinexhaust air through the cabin exhaust air branch line, downstream of thecabin exhaust air turbine, via a cabin air dump valve.
 14. The methodaccording to claim 13, further comprising at least one of the steps of:guiding a flow of bleed air through a backup line branching off from thebleed air supply line, downstream of the condenser, the flow of bleedair through the backup line being controlled via a backup valve arrangedin the backup line, and guiding a flow of hot bleed air through a bleedair branch line branching off from the bleed air supply line, whereinthe flow of hot bleed air through the bleed air branch line iscontrolled via a hot air regulation valve.
 15. The method according toclaim 12, wherein at least one of bleed air flowing through the bleedair supply line and cabin exhaust air flowing through the cabin exhaustair line is guided into the ram air channel, wherein cabin exhaust airis supplied to the ram air channel via an ejector disposed in the ramair channel configured to inject cabin exhaust air flowing through thecabin exhaust air line into the ram air channel and which is driven bybleed air exiting the bleed air supply line.